Backing up computers is an important task in the everyday operation of computer systems. Backup may be required for retrieval of information in cases of human errors, technical failures, upgrades, or the like, as well as for legal or operational reasons.
A disk, also referred to as a volume, in a mainframe (MF) operating system, has a proprietary architecture called Count Key Data (CKD) that dictates the disk name, such that the disk name is part of its architecture and is not dictated by the MF operating system. The volume can be in either online state or in offline state. In online state, the operating system and applications can access the disk for reading and/or writing data. When in offline state, no application can read or write information from the disk.
Each disk is identified by a 6-character volume serial number, referred to as a volume name. The MF operating system does not allow more than one volume with the same name to be in online state at the same time. Trying to bring online a disk with a duplicate volume name, makes the second disk declared as a duplicate and placed back in offline state.
In mainframe (MF) systems, backing up a disk, for example, using the DUMP FULL command of the Data Facility Storage Management Subsystem Data Set Services (DFSMSdss) system utility requires the volume to be in an online state.
Backup of a volume may take significant amount of time, for example a few hours. This limits the ability of an MF operator to create frequent backups, since the time it takes to create a backup may be longer than the required gap between backups. For example, an organization may require that a backup is created every two hours, such that in case of failure no more than two hours of work are lost. However, if creating a backup takes four hours to complete, such requirement cannot be complied with.
Additionally, the long time required for backing up a volume may also lead to the creation of low-integrity backups, as some of the data may have changed between when a backup started and when it ended, thus possibly creating inconsistency.
To overcome the problems of insufficient frequency of backup copies and inconsistent backups, a common practice is to create an identical copy of one or more volumes within the same disk storage system before initiating a backup for the volumes. The identical copy within the same storage system is referred to as a snapshot. Since snapshots are volume copies created within the same storage system instead of on an external storage, they are much faster to create in compare to backup copies, for example a few seconds, thus providing consistency.
However, due to the large storage capacity required for keeping snapshots, relying solely on snapshots as a backup mechanism does not allow keeping sufficient number of backup versions to provide sufficiently frequent recovery points. Therefore, it is common to combine snapshots with backups, such that the snapshots are used to create consistent copies in a timely manner, while backup of the snapshot copies is used to create multiple backup versions that can be stored on less expensive storage media such as cloud storage or tapes.
However, a snapshot volume behaves exactly as any other volume in a mainframe system. In order to initiate a backup of a snapshot volume, the snapshot has to be in an online state. As detailed above, the snapshot, which is an identical copy of the original volume, cannot be brought online without putting the original volume in offline state, since their names are identical.